The invention relates generally to syringes and more particularly to an improved syringe having a piston/piston rod assembly that is shiftable within a casing.
Such a syringe is disclosed in Netherlands patent application 90 00 487. Syringes of this type typically draw in liquid from a storage bottle that is sealed by a pierceable stopper. The piston/piston rod assembly is packaged in the extended condition. The air present in the syringe is driven into the bottle when the needle is inserted into the bottle, so that, upon subsequent withdrawal of the needle, the drawing in of liquid is not impaired by the lower pressure in the bottle. Prior to injection of the liquid into the patient, the remaining air in the syringe is ejected by directing the needle upwards. As the piston is completely pressed inwards during injection, coupling with the needle foot takes place.
A syringe thus made has the advantage that the air in the prepacked syringe may be made sufficiently sterile to avoid contamination of the contents of the bottle. A disadvantage is that the prepacked syringe has a considerable length, so that such prepacked syringes take up much space. Another disadvantage occurs in the event of incorrect operation of this syringe, whereby the coupling of the piston with the needle foot can take place prematurely. A further disadvantage is that the stroke limiting means have a relatively complicated shape, which increases manufacturing cost. Since such syringes are meant to be disposable, costs must be limited as much as possible.